The main goal of this project is to identify mechanisms of long-term synaptic plasticity in the auditory midbrain. Specifically, we intend to elucidate the role of feedback from the Optic Tectum (OT, equivalent to the mammalian superior colliculus) in modulating synaptic strength in the inferior colliculus. This visual feedback has been implicated in experience-dependent learning in both birds and mammals. We have obtained evidence of a synapse-specific long-term depression (LTD) of synaptic inputs onto neurons of the external nucleus of the inferior colliculus (EX) in chicken. This LTD is triggered by high-frequency stimulation of the afferent pathway and requires metabotropic glutamate receptor (mGluR), presynaptic N- methyl-D-aspartate receptor (NMDAR) and cannabinoid 1 receptor (CB1R) activation (Penzo and Pena 2008). We have collected preliminary data indicating that OT facilitates LTD induction at lower frequencies, possibly through the direct activation of mGluRs. Furthermore, we hypothesize that mGluR activation leads to retrograde glutamate release and activates presynaptic NMDARs. Here, we will address two main questions: the role of glutamate receptors in synaptic plasticity in the auditory midbrain as well as OT modulation of synaptic strength in the inferior colliculus. In light of the preliminary results the propose methodology have proven to be a plausible way of taking synaptic plasticity into the context of learning and behavior. Therefore, the knowledge gathered from this study will take us a step further to understanding the bases of learning and memory.